forked from lijiext/lammps
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HTML
262 lines
10 KiB
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<title>mass command — LAMMPS documentation</title>
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<a href="Manual.html" class="icon icon-home"> LAMMPS
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<ul>
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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_start.html">2. Getting Started</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_commands.html">3. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_packages.html">4. Packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_howto.html">6. How-to discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_example.html">7. Example problems</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_perf.html">8. Performance & scalability</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_tools.html">9. Additional tools</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_modify.html">10. Modifying & extending LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_python.html">11. Python interface to LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li><a href="Manual.html">Docs</a> »</li>
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<li>mass command</li>
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<a href="http://lammps.sandia.gov">Website</a>
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<a href="Section_commands.html#comm">Commands</a>
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<div itemprop="articleBody">
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<div class="section" id="mass-command">
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<span id="index-0"></span><h1>mass command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">mass</span> <span class="n">I</span> <span class="n">value</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>I = atom type (see asterisk form below)</li>
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<li>value = mass</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">mass</span> <span class="mi">1</span> <span class="mf">1.0</span>
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<span class="n">mass</span> <span class="o">*</span> <span class="mf">62.5</span>
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<span class="n">mass</span> <span class="mi">2</span><span class="o">*</span> <span class="mf">62.5</span>
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>Set the mass for all atoms of one or more atom types. Per-type mass
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values can also be set in the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> data file
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using the “Masses” keyword. See the <a class="reference internal" href="units.html"><span class="doc">units</span></a> command for
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what mass units to use.</p>
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<p>The I index can be specified in one of two ways. An explicit numeric
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value can be used, as in the 1st example above. Or a wild-card
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asterisk can be used to set the mass for multiple atom types. This
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takes the form “*” or “<em>n” or “n</em>” or “m*n”. If N = the number of
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atom types, then an asterisk with no numeric values means all types
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from 1 to N. A leading asterisk means all types from 1 to n
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(inclusive). A trailing asterisk means all types from n to N
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(inclusive). A middle asterisk means all types from m to n
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(inclusive).</p>
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<p>A line in a <a class="reference internal" href="read_data.html"><span class="doc">data file</span></a> that follows the “Masses”
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keyword specifies mass using the same format as the arguments of the
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mass command in an input script, except that no wild-card asterisk can
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be used. For example, under the “Masses” section of a data file, the
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line that corresponds to the 1st example above would be listed as</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mi">1</span> <span class="mf">1.0</span>
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</pre></div>
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</div>
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<p>Note that the mass command can only be used if the <a class="reference internal" href="atom_style.html"><span class="doc">atom style</span></a> requires per-type atom mass to be set.
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Currently, all but the <em>sphere</em> and <em>ellipsoid</em> and <em>peri</em> styles do.
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They require mass to be set for individual particles, not types.
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Per-atom masses are defined in the data file read by the
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command, or set to default values by the
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<a class="reference internal" href="create_atoms.html"><span class="doc">create_atoms</span></a> command. Per-atom masses can also be
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set to new values by the <a class="reference internal" href="set.html"><span class="doc">set mass</span></a> or <a class="reference internal" href="set.html"><span class="doc">set density</span></a>
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commands.</p>
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<p>Also note that <a class="reference internal" href="pair_eam.html"><span class="doc">pair_style eam</span></a> and <a class="reference internal" href="pair_bop.html"><span class="doc">pair_style bop</span></a> commands define the masses of atom types in their
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respective potential files, in which case the mass command is normally
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not used.</p>
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<p>If you define a <a class="reference internal" href="atom_style.html"><span class="doc">hybrid atom style</span></a> which includes one
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(or more) sub-styles which require per-type mass and one (or more)
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sub-styles which require per-atom mass, then you must define both.
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However, in this case the per-type mass will be ignored; only the
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per-atom mass will be used by LAMMPS.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>This command must come after the simulation box is defined by a
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a>, <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>, or
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<a class="reference internal" href="create_box.html"><span class="doc">create_box</span></a> command.</p>
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<p>All masses must be defined before a simulation is run. They must also
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all be defined before a <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> or <a class="reference internal" href="fix_shake.html"><span class="doc">fix shake</span></a> command is used.</p>
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<p>The mass assigned to any type or atom must be > 0.0.</p>
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<p><strong>Related commands:</strong> none</p>
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<p><strong>Default:</strong> none</p>
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</div>
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<p>
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© Copyright 2013 Sandia Corporation.
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